Embodiments of the invention relate to ultrasound imaging, and more particularly to non-invasive methods for detecting regions of altered stiffness.
Tissue stiffness is a known marker of disease. For example, some cancerous tissues are stiffer than the normal surrounding tissues. Treatments for certain conditions, such as ablation, also create stiffer regions of tissue. Significant change in tissue stiffness can occur without a related change in ultrasound echogenicity. Quantitative measurements of stiffness would be useful clinically in the diagnosis of fibrosis and steatosis to identify fibrous liver, for example. Further, detecting stiffness can also help in finding tumors, some of which are not visible in conventional ultrasound imaging.
For these reasons, it is clinically useful to have a way of visualizing the stiffness of tissue. There are numerous methods for making such images using ultrasound. Most of these methods involve moving the tissue and tracking the motion or displacement of the tissue. In one method, the tissue is compressed by the sonographer pushing with the ultrasound probe, and the elastic response of the tissue is measured. In another method, tissue motion is created by vibrating the tissue at a low frequency with an external shaker. In other methods, radiation force is employed to move the tissue. Acoustic radiation force impulse (ARFI) ultrasound imaging is being used to detect areas having altered stiffness. The basic idea of ARFI is to push the tissue with acoustic radiation and then use tracking techniques to detect the motion caused by the acoustic radiation.
It is known that, the stress-strain or equivalently the force-displacement relationship for healthy and diseased tissues are generally non-linear. The non-linear response of the tissue may provide additional information about the tissue that could improve the detection of cancer or other clinical conditions. For example, invasive ductal carcinoma (IDC) and normal glandular breast tissue have very different non-linear stress-strain relationships. The IDC becomes increasingly stiffer as the applied force is increased. The healthy glandular tissue also becomes stiffer as the applied force increases, but the slope of the curve for IDC is much steeper.